The present invention relates to an inspection device and the like for an active matrix organic light emitting diode (OLED) panel, and more specifically to an inspection device for conducting performance inspection of a thin film transistor (TFT) array prior to an OLED formation process.
An OLED (also referred to as organic electro luminescence (EL)) is for conducting a direct current on a fluorescent organic compound which is excited by application of an electric field, and thereby causing light emission of the compound. The OLED is drawing attention as a next-generation display device in terms of low-profileness, a wide view angle, and a wide gamut, etc. Whereas a driving method for the OLED includes a passive type and an active type, the active type is suitable for achieving a large-screen and high-definition display in light of aspects involving a material, a life, and crosstalks. This active type requires thin film transistor (TFT) driving, and a TFT array applying low-temperature polysilicon or amorphous silicon (a-Si) is drawing attention for this use.
There have been a conventional inspection method for a TFT array in a liquid crystal display (LCD). The method is configured to observe electric charges accumulated in a pixel capacitor by use of an integration circuit after writing a voltage in the pixel capacitor and thereby to inspect whether the voltage is written properly (see Patent Reference 1, for example). Meanwhile, there is disclosed a method to optically inspect writing in a pixel capacitor by use of a photoelectric element (see Patent Reference 2, for example). Moreover, there is also disclosed a method of inspecting writing by means of measuring electric potential of a pixels electrode simultaneously with conducting an electric current on the pixel electrode by an electron beam.
[Patent Reference 1]
U.S. Pat. No. 5,179,345 (Pages 3 to 5, FIG. 2)
[Patent Reference 2]
U.S. Pat. No. 4,983,911 (Pages 2 to 4, FIGS. 1 to 3)
Now, description will be made on comparison between an active matrix OLED (AMOLED) and an active matrix liquid crystal display (AMLCD). FIGS. 21A and 21B are diagrams for comparing and explaining pixel circuits in the AMOLED and the AMLCD. FIG. 21A shows a pixel circuit of the AMOLED and FIG. 21B shows a pixel circuit of the AMLCD. In FIG. 21B, the pixel circuit of a TFT array is formed by a TFT 310 which is connected to a data line (Data) and a gate line (Gate) Meanwhile, in the AMOLED shown in FIG. 21A, a driving TFT 302 which is an open drain driving transistor is connected adjacently to a pixel capacitor of a circuit similar to the one shown in FIG. 21B, and an OLED 301 which is a light emitting element is connected to the driving TFT 302.
The AMLCD can change gradation only by generating a voltage on the TFT 310. By contrast, in the AMOLED, luminance of the OLED 301 is changed in response to a value of an electric current flowing thereon when a predetermined voltage is applied to the driving TFT 302. Unevenness may be caused to threshold voltages Vth of these driving TFTs 302 even if a process is adjusted. When the threshold voltages are uneven, the electric currents flowing on the TFTs may vary even if the same voltage is applied thereto, and uneven luminance may occur. Accordingly, in the performance inspection of a TFT array for an AMOLED panel, in addition to inspection of open/short defects in the wiring, it is important to inspect whether characteristics of the driving TFTs 302 for driving the OLEDs 301 are uniform over the entire panel. Such inspection of uniformity corresponds to confirmation of uniformity in the threshold voltages Vth among the driving TFTs 302 on the panel owing to functions of compensation circuits for the driving TFTs 302.
Here, in order to reduce manufacturing costs of the current AMOLED panels, it is necessary to carry out a performance test on the independent TFT array and forward only a non-defective product to a subsequent process. It is desired to measure the threshold voltage Vth of the driving TFT 302 prior to mounting the OLED 301 in the manufacture of the AMOLED panel due to the reasons that: a product yield of the current TFT arrays for the AMOLED panels is not sufficiently high; raw material costs of the OLED 301 are high; a process for forming the OLED 301 occupies relatively a long time in the entire manufacturing process; and so on. Moreover, prior to mounting the OLED 301, it is required to confirm an etching condition of a pixel electrode subject to etching so that the TFT array having an etching defect is removed before forming the OLED 301.
However, in the independent TFT array, the OLED which is a constituent of the pixel circuit is not mounted, and the driving TFT is set to an open-drain state. That is, in the process prior to mounting the OLED, the OLED 301 indicated by broken lines in FIG. 21A is not connected and a normal circuit is not therefore established. Accordingly, it is basically impossible to conduct an electric current on the driving TFT 302, and it is not possible to carry out the performance inspection of the Vth compensation circuits, the performance inspection of the TFT array, or the inspection of patterning conditions of the pixel electrode if nothing is done.
Patent References 1 and 2 described above solely show the methods of inspecting the pixel circuit of the TFT array for the AMLCD as shown in FIG. 21B and do not possess a mechanism for supplying an electric current to the driving TFT 302 shown in FIG. 21A. As a result, it is not possible to perform the Vth measurement of the driving TFT 302 set to the open-drain state by use of the techniques disclosed in Patent References 1 and 2. Moreover, a method of carrying out inspection by means of connecting wiring to a drain of a driving TFT and conducting an electric current thereon is also conceivable. However, such a method seems unrealistic because there is a risk of contamination or damage of a TFT array by such wiring connection.